One method of coupling bonding pads of an integrated circuit chip to a package is by forming a metal bump (e.g., tin-lead (Sn--Pb) solder bump) on each pad and soldering the substrate to a package. This technology is sometimes referred to as controlled collapse chip connection (C4).
Improvements in the area of bump metallization include incorporating an under bump metallization (UBM) layer onto a chip. A UBM layer acts as a barrier preventing elements from diffusing between a pad on a substrate and the bump that connects a bump chip and its packaging substrate. Additionally, a UBM layer improves adhesion between the bump and the pad on a substrate. Moreover, a UBM layer may act as a wetting layer that ensures improved chip joint properties between a solder-based bump and the UBM layer. These advantages apply to a UBM layer comprising a two layer structure A/B--C, in which A is, for example, a non-refractory metal such as gold or nickel and B--C is a binary metal alloy such as titanium/tungsten (Ti/W), or a three layer structure A/B/C in which A is a nonrefractory metal and B and C are refractory metals.
Refractory metals and their nitrides, such as titanium (Ti) and titanium nitride (TiN) are widely used as adhesion promoter and diffusion barrier layers in UBM. A refractory metal nitride layer such as TiN can be formed by depositing TiN via physical sputtering or chemical vapor deposition or annealing refractory metal in an N.sub.2 or NH.sub.3 ambient at temperatures up to 700.degree. C. (However, high temperature back-end processing can lead to serious reliability related failure, especially in a chip using copper-based interconnects.
Several other disadvantages exist with processes known in the art. One problem is that inter diffusion of elements occurs between a bump and a pad when a packaged chip operates at a higher temperature. This causes the UBM layer to react with tin (Sn) contained in solder to form intermetallic compounds that are mechanically and electrically harmful to chip joints. The interfacial reaction at the bump/UBM interface can lead to delamination of the UBM layer on a pad, inducing chip malfunction. Moreover, electromigration (EM) induced failure may result at the bump/UBM interface from using known methods due to "voids" forming near the interface of the solder bump materials such as tin-lead (Sn--Pb) and UBM materials when a high current is imposed on a bump at 100.degree. C. It is therefore desirable to simplify the process of forming UBM layers and generally improve the effectiveness of the UBM layer.